Automatic bias for dectectors of superheterodyne receivers



@Cit. 8, 1935. 3 w SEELEY 2,017fii AUTOMATIC BIAS FOR DETECTORS OF SUPERHETERODYNE RECEIVERS Filed Jun e 21, 1935 lNVENTO 1 Stan I W Seeizy BY A ORNEY- Patented Oct. 8 193$ PATENT OFFICE AUTOMATIC BIAS F SUPERHETEROD Stuart W. Seeley, Jackson,

R DECTECTORS 0F YNE RECEIVERS Mich, assignor, by

mesne assignments, to Radio Corporation of America, New York, N.

Delaware Y., a corporation of Application June 21, 1933, Serial No.676,814

1 Claim.

It is well known that if the feed-back coupling and other constants in a superheterodyne oscillator remain constant the voltage applied to the first detector from such oscillator increases with an increase in frequency. It is also well known that for optimum detector sensitivity it is necessary that the bias of such detector be always a certain fixed amount greater than the maximum peak swing of the oscillator energy applied to it. These conditions necessitate either costly measures for maintaining the oscillator applied voltage at constant value or varying the bias of the detector tube as the oscillator potential changes.

I have evolved a very simple method of maintaining the detector bias higher than the peak voltage of the oscillator signal by any desired fixed amount.

One object of the invention is to produce an automatic bias for a detector tube coupled to a superheterodyne oscillator.

Another object of the invention is to produce a bias on an intermediate frequency detector by a resistance drop in the grid circuit of the oscillator whereby the bias on the detector varies directly with the variation of voltage of the oscillator.

Other objects will appear in the following description, reference being had to the drawing in which:

The single figure of the drawing illustrates circuit arrangements in which the invention is employed.

Referring to the drawing:

The oscillator I has its coil L1 in the tuned plate circuit 2 coupled to its grid coil 3 by a predetermined mutual inductance M1 between coils L1 and 3. The grid coil 3 is connected to the filament lead 4 through a resistance R1. This resistance has a bypass condenser C1 to filter out the alternating component of the drop around the resistance. One end of the tuned loop 2 is connected to the plate of tube 1 and the other end is connected to the positive side of a plate supply not shown, the negative of which is connected to ground G. The grid 1 of the first detector tube 8 is coupled through coils 9 and ID with the tuned loop of the oscillator plate circuit. The coils 9 and ID are tuned by a variable condenser which may be unicontrolled with the condenser l2 of the oscillator plate circuit, these being constructed so that there will always be a desired diiierence between the oscillator frequency and the signal frequency feeding into the detector coil 9 through primary 13. The resistance R2 couples the grid 1 of the detector to the negative end of resistance R1 to produce the negative bias on the grid. The plate iii of the detector is connected to loop lttuned at the factory or service station to the intermediate frequency and this tuned loop is coupled to subsequent circuits which form no part of the invention and are not illustrated or described. Condenser C2 filters out any alternating component passing through the resistance R2.

In operation the signal from coil I3 is fed into 10 coil 9 of the first detector tube so that the voltage is applied between the grid 1 and ground G. The oscillator voltage is induced into coil it of this grid circuit so that there is a combination between the voltage of the two frequencies and a voltage of intermediate frequency is produced in a well known way. Due to feed-back between the coils L1 and 3 in the oscillator circuit rectified current flows in the grid circuit through resistance R1, the alternating component being shunted through bypass condenser C1. Since the grid circuit of the detector tube is connected through resistance R to the negative end of the resistance R1, a negative bias is produced on the grid 1, as desired. If a station of a higher frequency is tuned in by manipulation of the condensers H and I2 the voltage applied through the modulator coupling M1, M2 increases, necessitating an increase in the grid bias of the detector tube for best operation. Since the voltage introduced in coil 3 is increased, there will be increased current through resistance R1, which increases the negative bias on the detector tube in the proportion desired to give best operation. It will thus be seen that as the set is operated throughout the desired frequency range the detector grid bias is always maintained at the right value for proper detection of the intermediate frequency produced by the reaction of the oscillator with the incoming signal.

It may be stated that good results have been obtained for oscillator frequencies between 725 k. c. to 1675 k. c. with the following values:

M =20 microhenries. M2=7 microhenries. L1=210 microhenries. R1=50,000 ohms. R2=250,O00 ohms. 01:.002 microfarads. 02:.2 microfarads.

The above values are given only by way of example and variations of course may be made in practice.

It will be apparent that various modifications 2 may be made without departing from the spirit of my invention.

Having described. my invention, what I claim In superheterodyne receiving sets, a vacuum tube oscillator having substantially a constant feed-back coupling tending to vary the oscillator voltage directly with its frequency of oscillations, said oscillator having a control electrode, a cathode and an anode, a resistance in the control electrode-cathode circuit of the oscillator having a D. C. potential drop proportional to the voltage in said circuit a first-detector tube for rectifying the beats produced by the oscillator and the signal frequencies, said detector tube having a control electrode, a cathode and an anode, means to connect the control electrode and the cathode of said first detector tube respectively to the negative and positive ends of said resistance to produce a negative bias on the control electrode of the first detector tube varying directly with the voltage of the oscillator whereby the optimum value of negative bias is 10 automatically produced for each frequency of oscillation.

STUART W. SEELEY. 

